1. Field of the Invention
The present invention generally relates to an electronic camera apparatus and, more particularly, to an electronic camera apparatus which can effectively pick up an image of a target object (to be photographed) having a wide dynamic range and allows it to be displayed on an external image monitor.
2. Description of the Related Art
Various types of solid-state image pickup elements, such as CCD image sensors and MOS type image sensors, have recently been developed. Consequently, various types of electronic cameras for electronically picking up an image of a target object by using these solid-state image pickup elements have been developed as, e.g., video cameras and electronic still cameras. An electronic camera of this type is designed to record an image signal (image pickup signal) of a target object, electronically picked up by a solid-state image pickup element, in, e.g., a magnetic recording medium such as a so-called video tape or a floppy disk, or an IC memory card and to allow it to be displayed on an image monitor such as a TV receiver.
The dynamic range of a video signal which can be displayed on an image monitor of the TV receiver or the like is 40 dB at best. For this reason, the image pickup capacity (dynamic range) of a solid-state image pickup element is generally set to be about 50 dB.
In contrast to this, the dynamic range of a target object to be picked up by the solid-state image pickup element (electronic camera) often reaches 80 dB. That is, the dynamic range (signal level width) of a target object often greatly exceeds a dynamic range in which an image display can be performed by an external image monitor, and image pickup processing can be performed by a solid-state image pickup element.
When a target object having such a wide dynamic range is to be electronically picked up by the solid-state image pickup element without any modifications, saturation of the solid-state image pickup element occurs at a high signal level portion (high-brightness portion), thus causing so-called white omissions. In contrast to this, if this saturation is suppressed, a low signal level portion (low-brightness portion) suffers from underexposure, thus causing so-called blackening. That is, a signal component which falls outside the dynamic range of a solid-state image pickup element or an image monitor cannot be picked up or displayed.
As described above, a target object often has a dynamic range wider than that of a solid-state image pickup element. In the conventional technique, if such a target object is electronically picked up by the solid-state image pickup element without any modifications, object data at a high-level or low-level portion is impaired. More specifically, if a target object having a wide dynamic range is electronically picked up by a solid-state image pickup element, the dynamic range of the target object is restricted by the dynamic range of the solid-state image pickup element or the image monitor. As a result, only an image signal (video signal) having a limited dynamic range can be obtained. Especially in a solid-state image pickup element, an essential problem is posed, wherein vertical stripes are generated on a display screen due to fixed pattern noise.